786 research outputs found
Spitzer IRS Observations of the Galactic Center: Shocked Gas in the Radio Arc Bubble
We present Spitzer IRS spectra (R ~600, 10 - 38 micron) of 38 positions in
the Galactic Center (GC), all at the same Galactic longitude and spanning
plus/minus 0.3 degrees in latitude. Our positions include the Arches Cluster,
the Arched Filaments, regions near the Quintuplet Cluster, the ``Bubble'' lying
along the same line-of-sight as the molecular cloud G0.11-0.11, and the diffuse
interstellar gas along the line-of-sight at higher Galactic latitudes. From
measurements of the [O IV], [Ne II], [Ne III], [Si II], [S III], [S IV], [Fe
II], [Fe III], and H_2 S(0), S(1), and S(2) lines we determine the gas
excitation and ionic abundance ratios. The Ne/H and S/H abundance ratios are ~
1.6 times that of the Orion Nebula. The main source of excitation is
photoionization, with the Arches Cluster ionizing the Arched Filaments and the
Quintuplet Cluster ionizing the gas nearby and at lower Galactic latitudes
including the far side of the Bubble. In addition, strong shocks ionize gas to
O^{+3} and destroy dust grains, releasing iron into the gas phase (Fe/H ~ 1.3
times 10^{-6} in the Arched Filaments and Fe/H ~ 8.8 times 10^{-6} in the
Bubble). The shock effects are particularly noticeable in the center of the
Bubble, but O is present in all positions. We suggest that the shocks
are due to the winds from the Quintuplet Cluster Wolf-Rayet stars. On the other
hand, the H_2 line ratios can be explained with multi-component models of warm
molecular gas in photodissociation regions without the need for H_2 production
in shocks.Comment: 51 pages, 17 figures To be published in the Astrophysical Journa
HST NICMOS Observations of the Polarization of NGC 1068
We have observed the polarized light at 2 micron in the center of NGC 1068
with HST NICMOS Camera 2. The nucleus is dominated by a bright, unresolved
source, polarized at a level of 6.0 pm 1.2% with a position angle of 122degr pm
1.5degr. There are two polarized lobes extending up to 8'' northeast and
southwest of the nucleus. The polarized flux in both lobes is quite clumpy,
with the maximum polarization occurring in the southwest lobe at a level of 17%
when smoothed to 0.23'' resolution. The perpendiculars to the polarization
vectors in these two lobes point back to the intense unresolved nuclear source
to within one 0.076'' Camera 2 pixel, thereby confirming that this is the
illuminating source of the scattered light and therefore the probable AGN
central engine. Whereas the polarization of the nucleus is probably caused by
dichroic absorption, the polarization in the lobes is almost certainly caused
by scattering, with very little contribution from dichroic absorption. Features
in the polarized lobes include a gap at a distance of about 1'' from the
nucleus toward the southwest lobe and a ``knot'' of emission about 5''
northeast of the nucleus. Both features had been discussed by ground-based
observers, but they are much better defined with the high spatial resolution of
NICMOS. The northeast knot may be the side of a molecular cloud that is facing
the nucleus, which cloud may be preventing the expansion of the northeast radio
lobe at the head of the radio synchrotron-radiation-emitting jet. We also
report the presence of two ghosts in the Camera 2 polarizers. These had not
been detected previously (Hines et al. 2000) because they are relatively faint
and require observations of a source with a large dynamic range.Comment: 17 pages, 4 figure
First observations with CONDOR, a 1.5 THz heterodyne receiver
The THz atmospheric windows centered at roughly 1.3 and 1.5~THz, contain
numerous spectral lines of astronomical importance, including three high-J CO
lines, the N+ line at 205 microns, and the ground transition of para-H2D+. The
CO lines are tracers of hot (several 100K), dense gas; N+ is a cooling line of
diffuse, ionized gas; the H2D+ line is a non-depleting tracer of cold (~20K),
dense gas. As the THz lines benefit the study of diverse phenomena (from
high-mass star-forming regions to the WIM to cold prestellar cores), we have
built the CO N+ Deuterium Observations Receiver (CONDOR) to further explore the
THz windows by ground-based observations. CONDOR was designed to be used at the
Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared
Astronomy (SOFIA). CONDOR was installed at the APEX telescope and test
observations were made to characterize the instrument. The combination of
CONDOR on APEX successfully detected THz radiation from astronomical sources.
CONDOR operated with typical Trec=1600K and spectral Allan variance times of
30s. CONDOR's first light observations of CO 13-12 emission from the hot core
Orion FIR4 (= OMC1 South) revealed a narrow line with T(MB) = 210K and
delta(V)=5.4km/s. A search for N+ emission from the ionization front of the
Orion Bar resulted in a non-detection. The successful deployment of CONDOR at
APEX demonstrates the potential for making observations at THz frequencies from
ground-based facilities.Comment: 4 pages + list of objects, 3 figures, to be published in A&A special
APEX issu
R-matrix Floquet theory for laser-assisted electron-atom scattering
A new version of the R-matrix Floquet theory for laser-assisted electron-atom
scattering is presented. The theory is non-perturbative and applicable to a
non-relativistic many-electron atom or ion in a homogeneous linearly polarized
field. It is based on the use of channel functions built from field-dressed
target states, which greatly simplifies the general formalism.Comment: 18 pages, LaTeX2e, submitted to J.Phys.
Sequence determinants in human polyadenylation site selection
BACKGROUND: Differential polyadenylation is a widespread mechanism in higher eukaryotes producing mRNAs with different 3' ends in different contexts. This involves several alternative polyadenylation sites in the 3' UTR, each with its specific strength. Here, we analyze the vicinity of human polyadenylation signals in search of patterns that would help discriminate strong and weak polyadenylation sites, or true sites from randomly occurring signals. RESULTS: We used human genomic sequences to retrieve the region downstream of polyadenylation signals, usually absent from cDNA or mRNA databases. Analyzing 4956 EST-validated polyadenylation sites and their -300/+300 nt flanking regions, we clearly visualized the upstream (USE) and downstream (DSE) sequence elements, both characterized by U-rich (not GU-rich) segments. The presence of a USE and a DSE is the main feature distinguishing true polyadenylation sites from randomly occurring A(A/U)UAAA hexamers. While USEs are indifferently associated with strong and weak poly(A) sites, DSEs are more conspicuous near strong poly(A) sites. We then used the region encompassing the hexamer and DSE as a training set for poly(A) site identification by the ERPIN program and achieved a prediction specificity of 69 to 85% for a sensitivity of 56%. CONCLUSION: The availability of complete genomes and large EST sequence databases now permit large-scale observation of polyadenylation sites. Both U-rich sequences flanking both sides of poly(A) signals contribute to the definition of "true" sites. However, the downstream U-rich sequences may also play an enhancing role. Based on this information, poly(A) site prediction accuracy was moderately but consistently improved compared to the best previously available algorithm
Hyperspherical partial wave calculation for double photoionization of the helium atom at 20 eV excess energy
Hyperspherical partial wave approach has been applied here in the study of
double photoionization of the helium atom for equal energy sharing geometry at
20 eV excess energy. Calculations have been done both in length and velocity
gauges and are found to agree with each other, with the CCC results and with
experiments and exhibit some advantages of the corresponding three particle
wave function over other wave functions in use.Comment: 11 pages, 1 figure, submitted to J. Phys B: At. Mol. Opt. Phys; v2 -
revised considerably, rewritten using ioplatex clas
Symmetrized complex amplitudes for He double photoionization from the time-dependent close-coupling and exterior complex scaling methods
Symmetrized complex amplitudes for the double photoionization of helium are computed by the time-dependent close-coupling and exterior complex scaling methods, and it is demonstrated that both methods are capable of the direct calculation of these amplitudes. The results are found to be in excellent agreement with each other and in very good agreement with results of other ab initio methods and experiment
Hubble Space Telescope NICMOS Polarization Measurements of OMC-1
We present 2micron polarization measurements of positions in the BN region of
the Orion Molecular Cloud (OMC-1) made with NICMOS Camera 2 (0.2'' resolution)
on HST. Our results are as follows: BN is sim 29% polarized by dichroic
absorption and appears to be the illuminating source for most of the nebulosity
to its north and up to sim 5'' to its south. Although the stars are probably
all polarized by dichroic absorption, there are a number of compact, but
non-point-source, objects that could be polarized by a combination of both
dichroic absorption and local scattering of star light. We identify several
candidate YSOs, including an approximately edge-on bipolar YSO 8.7'' east of
BN, and a deeply-embedded variable star. Additional strongly polarized sources
are IRc2-B, IRc2-D, and IRc7, all of which are obviously self-luminous at
mid-infrared wavelengths and may be YSOs. None of these is a reflection nebula
illuminated by a star located near radio source I, as was previously suggested.
Other IRc sources are clearly reflection nebulae: IRc3 appears to be
illuminated by IRc2-B or a combination of the IRc2 sources, and IRc4 and IRc5
appear to be illuminated by an unseen star in the vicinity of radio source I,
or by Star n or IRc2-A. Trends in the magnetic field direction are inferred
from the polarization of the 26 stars that are bright enough to be seen as
NICMOS point sources. The most polarized star has a polarization position angle
different from its neighbors by sim 40^o, but in agreement with the grain
alignment inferred from millimeter polarization measurements of the cold dust
cloud in the southern part of OMC-1.Comment: 41 pages, 8 figures, 4 tables, to appear in The Astrophysical Journa
‘O sibling, where art thou?’ – a review of avian sibling recognition with respect to the mammalian literature
Avian literature on sibling recognition is rare compared to that developed by mammalian researchers. We compare avian and mammalian research on sibling recognition to identify why avian work is rare, how approaches differ and what avian and mammalian researchers can learn from each other. Three factors: (1) biological differences between birds and mammals, (2) conceptual biases and (3) practical constraints, appear to influence our current understanding. Avian research focuses on colonial species because sibling recognition is considered adaptive where ‘mixing potential’ of dependent young is high; research on a wider range of species, breeding systems and ecological conditions is now needed. Studies of acoustic recognition cues dominate avian literature; other types of cues (e.g. visual, olfactory) deserve further attention. The effect of gender on avian sibling recognition has yet to be investigated; mammalian work shows that gender can have important influences. Most importantly, many researchers assume that birds recognise siblings through ‘direct familiarisation’ (commonly known as associative learning or familiarity); future experiments should also incorporate tests for ‘indirect familiarisation’ (commonly known as phenotype matching). If direct familiarisation proves crucial, avian research should investigate how periods of separation influence sibling discrimination. Mammalian researchers typically interpret sibling recognition in broad functional terms (nepotism, optimal outbreeding); some avian researchers more successfully identify specific and testable adaptive explanations, with greater relevance to natural contexts. We end by reporting exciting discoveries from recent studies of avian sibling recognition that inspire further interest in this topic
Global energy governance : a review and research agenda
Over the past few years, global energy governance (GEG) has emerged as a major new field of enquiry in international studies. Scholars engaged in this field seek to understand how the energy sector is governed at the global level, by whom and with what consequences. By focusing on governance, they broaden and enrich the geopolitical and hard-nosed security perspectives that have long been, and still are, the dominant perspectives through which energy is analysed. Though still a nascent field, the literature on GEG is thriving and continues to attract the attention of a growing number of researchers. This article reviews the GEG literature as it has developed over the past 10 years. Our aim is to highlight both the progress and limitations of the field, and to identify some opportunities for future research. The article proceeds as follows. First, it traces the origins of the GEG literature (section “Origins and roots of GEG research”). The subsequent sections deal with the two topics that have received the most attention in the GEG literature: Why does energy need global governance (section “The goals and rationale of global energy governance”)? And, who governs energy (section “Mapping the global energy architecture”)? We then address a third question that has received far less attention: How well or poor is energy governed (section “Evaluating global energy governance”)? In our conclusions (section “Conclusions and outlook”), we reflect on the current state of GEG, review recent trends and innovations, and identify some questions that warrant future consideration by scholars. This article is published as part of a thematic collection on global governance
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